Fix 3d Track Sliding With Tracker

Did a complex syntheyes track...the track is about 3 years old and it was handheld and a lot of blur in it.

Got very good results considering, but there is some small sliding issues in the footage...very slight...but visible to the trained eye.

I was hoping there would be a procedural way to fix this via the tracker.

My thinking is that I could perhaps track a rock near the sliding object and reference that as my baseline track...
Then track the object sliding, and compare the results with the baseline track and transform it in 2d to match...

Not quite sure how to achieve this...basically 1 want to track object 1 >>>> then track object 2 and subtract object 1's track to zero out camera motion so all that is left is the sliding transform value

This should work 2d only due to the nature of the shot...and retracking in syntheyes is not going to get me any better results as this is the best possible result that you can get from it.

You're on the right track (haha, sorry, pun intended).
You can do this with an expression modifier on a transform's center.
Point 1 is the 2D track of your rock near the sliding object.
Point 2 would be the track of your cg object which can be done in 2D but you can also use a locator in 3D space and use its coordinates.

Then as a formula in the point result tab enter p1x-p2x and p1y-p2y respectively.